A conventional mode field diameter conversion fiber comprising a length of an optical waveguide including a quartz glass cladding and a germanium-doped quartz glass core, wherein the core diameter varies along the optical waveguide, increasing towards its end, is disclosed, e.g., by K. Shiraishi, Y. Aizava, S. Kawakami in the reference titled as "Beam Expanding Fiber Using Thermal Diffusion of Dopant" in IEEE Journal of Lightwave Technology, 1990, vol.8, No.8, p.1151-1161. In the mode field diameter conversion fiber of the above reference, the longitudinal variation in a core diameter is provided owing to redistribution of the radial doping profile, which forms the refractive index structure of the optical waveguide, in the process of thermal diffusion of germanium.
The problem with the mode field diameter conversion fiber is a complicated technology caused by a small coefficient of germanium diffusion to quartz glass, resulting in a prolonged heat treatment of the optical waveguide, required to produce a mode field diameter conversion fiber. Apart from that, the diffusion occurs efficiently only at the temperature of 1600.degree. C. to 1800.degree. C. that is close to the melting point and, therefore, causes deformations in optical waveguides.
Closely approaching the claimed invention is a mode field diameter conversion fiber comprising a length of an optical waveguide including a quartz glass cladding and a doped quartz glass core, wherein the core diameter varies along the optical waveguide, increasing towards its end (see, e.g., U.S. Pat. No. 5,381,503, Int.Cl. G 02 B 6/10). In the prior art fiber, the core is initially doped with germanium and fluorine. Unlike germanium, fluorine reduces the quartz glass refractive index and additionally exhibits a greater thermal diffusion coefficient at the temperature of 1600.degree. C. to 1800.degree. C. As the result, when the optical waveguide with the double-doped core is heated, fluorine faster penetrates into the cladding, providing an efficient increase in the glass core refractive index and, therefore, a decreased mode field diameter.
A disadvantage of the above prior art is a complicated fabrication of the mode field diameter conversion fiber and a narrow range of mode field diameter variation, which is caused by a limited concentration of fluorine that can be introduced into the core along with germanium. Furthermore, the fluorine diffusion will unavoidably lead to the appearance, in such an optical waveguide, of regions with a decreased refractive index, which hampers coupling to conventional optical waveguides.
Described by O. Hill, Y. Fujii, D. C. Johnson and B. S. Kawasaki in the reference titled as "Photosensitivity in Optical Fiber Waveguides: Application to Reflection Filter Fabrication". Appl. Phys. Lett. 1978, Vol.32, No.10, p.647-649, is a method for locally changing the refractive index of an optical waveguide, involving subjecting the optical waveguide to external exposure. Change in the refractive index occurs due to the photorefractive effect and is caused by the presence of defects that give rise to the appearance of specific bands in the absorption spectrum of the optical waveguide core. In this case, a two-photon interaction takes place. In the optical waveguide, an incoming beam and a beam reflected from the end face interfere, causing a periodic change in the refractive index.
Disadvantages of the above prior art method include a sophisticated process, a small change (.DELTA..about.10.sup.-6) in the refractive index, impossibility to vary a period of the grating produced, high cost and complexity of operating the apparatus implementing the method.
Another method for locally changing the refractive index of an optical waveguide, involving subjecting the optical waveguide to external exposure, was disclosed by G. Meltz, W. W. Morey, W. H. Glen in the reference titled as "Formation of Bragg Gratings in Optical Fibers by a Transverse Holographic Method". Opt. Lett., 1989, vol.14, No.15, p.823-825. In accordance with the method, an optical waveguide is exposed to a laser radiation at a wavelength close to 240 nm. A grating is formed in the optical waveguide using an interference effect, by directing two beams at an angle .theta. to the optical waveguide surface. A period of the grating formed can be altered by varying the angle .theta..
The above prior art method, however, suffers from an elaborated technology of locally changing the refractive index of an optical waveguide.
K. O. Hill, F. Bilodean, B. Malo, D. C. Malo, D. C. Johnson, I. S. Kinner, "Efficient Mode Conversion in Telecommunication Fiber Using Externally Writing Gratings". Electron. Lett., 1990, vol.26, p.1270-1272, disclose a method for locally changing the refractive index of an optical waveguide, involving subjecting the optical waveguide to external exposure. In this method, a photosensitive optical waveguide is exposed, at the side surface, to ultraviolet laser radiation that has passed through a narrow slit. Within the exposure region, a photo-induced local increase in the refractive index occurs, which corresponds to writing a single groove of the grating. The exposure is then interrupted and the optical waveguide is axially displaced relative to the laser beam at a distance equal to the grating period, following which a next grating groove is written. By periodically repeating the optical waveguide exposure and axial displacement procedures, a periodical sequence of core regions having different refraction indices is formed, representing a grating.
However, the process of locally changing the refractive index is rather complicated owing to use of the ultraviolet laser radiation and by virtue of the fact that a number of photosensitive types of optical waveguides is limited. In addition, the employed ultraviolet lasers are relatively expensive, sophisticated in service, have insufficient reliability and their radiation is hazardous to human eye.
Closely approaching the present invention is a method for locally changing the refractive index of an optical waveguide, involving heating a length of an optical waveguide including a cladding and a core, each one being based on quartz glass and at least one of them being doped, providing a local thermal diffusion of the elements contained in the core to the cladding, and/or providing a local thermal diffusion of the elements contained in the cladding to the core. See, e.g. U.S. Pat. No. 5,381,503, Int.Cl. G 02 B 6/10. In this method, a length of an optical waveguide is heated in a resistance-heated furnace.
The double doping with germanium and fluorine significantly complicates the fabrication of optical waveguides for a mode field diameter conversion fiber. A further disadvantage of this method is that it is impossible to control the law of changing the core refractive index along the optical fiber axis, which is defined by the temperature distribution within the heating apparatus and cannot be modified in the course of the process. A further problem is that the optical waveguide undergoes deformation when heated, owing to relaxation of lateral stresses appearing when the optical waveguide is fixed.
A method for fabricating a preform for a quartz glass-based optical waveguide, including feeding a mixture of molecular gaseous agents containing atoms of oxygen and silicon into a substrate tube, and depositing the products of the reaction occurring in the mixture on the inner surface of the substrate tube, is described, e.g., in the Reference Book of Fiber Optic Communication Links, L. M. Andrushko, V. A. Voznesenskii, V. B. Katok et al.; under edition of S. V. Svechnikov and L. M. Andrushko, Kiev, Tekhnika, 1988, p.69.
However, fabrication of a preform from nitrogen-doped quartz glass is unfeasible under thermodynamic equilibrium conditions typical for this method.
A method for fabricating preforms for optical waveguides based on nitrogen-doped quartz glass, that is most closely approaching the claimed invention, includes feeding a mixture of molecular gaseous agents containing atoms of nitrogen, oxygen and silicon into a substrate tube, exciting a microwave discharge in the mixture and depositing products of the reaction occurring in the mixture on the inner surface of the substrate tube. See, e.g., E. M. Dianov, K. M. Golant, R. R. Kharpko, A. S. Kurkov, A. L. Tomashuk. "Low-Hydrogen Silicon Oxynitride Optical Fibers Prepared by SPCVD". IEEE Journal of Lightwave Technology, 1995, vol.13, No.7, p.1471-1474. The basis for this method is fabrication of an optical waveguide preform by plasma-enhanced synthesis of nitrogen-doped quartz glass layers on the inner wall of a quartz glass substrate tube by the SPCVD process.
The method, however, is technologically complicated since it does not employ process parameters in the preform synthesis, which would allow a reproducible control of the nitrogen concentration in the quartz glass, required to obtain a desired refractive index difference .DELTA.n between the optical waveguide core and cladding.